Action News reporters say hundreds of residents are waiting to cast their ballot at some locations.
Election officials say the Woodward Park Regional Library is the most popular voting center within Fresno County.
Fresno County Registrar of Voters James Kus says the long lines are in part due to people using California's same-day voter registration system.
Even when polling locations close at 8 pm, voters already in line at that time will be allowed to cast a ballot.
Officials say it will still take several days until they're able to count all of the vote-by-mail ballots that were dropped off and returned.
A Fresno County Sheriff's Office helicopter will help be part of the tabulation process.
At around 8:45 pm, the helicopter is expected to land with a memory card containing results from Coalinga area vote centers.
Click here to see every polling location in the Central Valley and the key races to watch this election night.